uwsgi: the php plugin needs to link with session.so

On NixOS the session.so php extension is a separate package (and not builtin
in libphp.so). But since the uwsgi php plugin uses the session
mechanisms, we need to link the plugin to that library too.

With this change uwsgipluginpy is hacked to take an additional UWSGICONFIG_PHP_LDFLAGS
environment variable and add it to its LDFLAGS, and then in the nix
expression the UWSGICONFIG_PHP_LDFLAGS is set to point to php.extensions.session

pkgs/servers/uwsgi/no-ext-session-php_session.h-on-NixOS.patch

@@ -0,0 +1,29 @@
+From 6b9b3559d8ce59eda6c5cd6f04224cebaaa5d0ea Mon Sep 17 00:00:00 2001
+From: =?UTF-8?q?=D0=94=D0=B0=D0=BC=D1=98=D0=B0=D0=BD=20=D0=93=D0=B5=D0=BE?=
+ =?UTF-8?q?=D1=80=D0=B3=D0=B8=D0=B5=D0=B2=D1=81=D0=BA=D0=B8?=
+ <gdamjan@gmail.com>
+Date: Tue, 8 Sep 2020 17:11:39 +0200
+Subject: [PATCH] no ext/session/php_session.h on NixOS
+
+on NixOS php_session.h is in its own package, and is not installed in
+ext/session/
+---
+ plugins/php/common.h | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/plugins/php/common.h b/plugins/php/common.h
+index 9bf1c069..be93f519 100644
+--- a/plugins/php/common.h
++++ b/plugins/php/common.h
+@@ -10,7 +10,7 @@
+ #endif
+ #include "ext/standard/info.h"
+ 
+-#include "ext/session/php_session.h"
++#include "php_session.h"
+ 
+ #include <uwsgi.h>
+ 
+-- 
+2.28.0
+